Ferrite powders, such as can be prepared, for example from barium, strontium or other metallic ferrites have been shaped and sintered to form many useful materials. Amongst the most useful of these materials are flexible permanent magnets which have been manufactured by dispersing ferrite particles or powders in a flexible plastic matrix.
It has been observed that for optimum utility, the particle size of the ferrite mixed in the plastic should be such that particle size is close to that of a single magnetic domain. In the case of powders of barium, strontium and similar ferrites the magnetic domain is approximately 1.mu.. Therefore the best products are prepared from powders in which the average particle diameter is about 1.mu.. Some variation is acceptable however, so that for particular applications the range of average particle size may be from about 1.mu. to 1.75.mu..
If the average particle diameter is appreciably below this value, kneading becomes more and more difficult in the usual equipment with the result that the dispersion of the ferrite in the plastic is poor, and it is difficult to prepare products containing optimum amount of magnetic material. As the average particle diameter increases above 1.mu., and expecially as it increases appreciably above 1.5.mu., the particles exceed a single magnetic domain and become multi-domain. This results in a reduction of both magnetic retentivity and energy product.
Magnetic retentivity or coercive force (H) is the magnetizing force that must be applied to a magnetic material in a direction opposite the residual induction in order to reduce the induction to zero. It is expressed in oersteds. For best products the value should be at least 2450 as .sub.I H.sub.C.
Energy product is the product of the magnetic induction B and magnetic retentivity expressed in gauss oersteds.
For flexible permanent magnets these values should be as high as possible. Magnets produced in accordance with the process of this invention, in addition to their other improved magnetic properties manifest a magnetic retentivity of at least 2450 oersteds.
It has proved difficult to pulverize ferrite particles to the desired average particle diameter for the production of optimized flexible magnets. Two procedures are presently employed, the dry method and the wet method. In the dry method the particles are pulverized without any added lubricant. A disadvantage of the dry method is that excessively long periods of time are required to produce particles of the desired size. The increased time adds to the cost of the operation. The problem has been attributed to the fact that certain amounts of ferrite powder as it forms, adheres to the internal surfaces or the crushing media of the machines, and dampens the crushing impact needed for pulverizing action.
In the wet method, pulverization promoters such as higher fatty acids and higher alcohols are employed. Stearic acid is especially popular. A difficulty with the method is that it is difficult to achieve the desired particle size. It is however, the preferred of the two methods.
Hard ferrite powder produced by pulverizing barium, strontium or similar ferrites generally manifest poor magnetic properties due to various mechanical stresses and distortions which develop in the powder during crushing. These are conventionally corrected by annealing at high temperature. The resulting annealed products are normally well adapted for use in flexible rubber or plastic magnets.
It has been observed that during the annealing process process powders produced using stearic or other higher fatty acid tend to sinter and adhere together during annealing. As a result the ferrite particles produced are larger than the desired magnetic domain size. As a result optimum magnetic characteristics are not developed in the particles themselves, or in flexible magnets employing them. A particular difficulty with the larger particles formed from sintered and fused grains is that they are not amenable to magnetic orientation such as is utilized in the manufacture of flexible magnets of the anisotropic type.